Automatic transmissions for apparatus in which torque has to be transmitted from an engine to an output are well known. The gear shift takes place in order to utilize the available torque from the engine in the best possible manner. Depending on the type of gearing used, the torque may be transmitted during gear shifts or the transmission of torque may be interrupted. The present invention concerns itself with transmissions in which torque is transmitted during the gear shift operation. In such known apparatus planetary gears are generally used. In order to change the gear ratio, hydraulically operable couplings or brakes are provided which drive or brake the sun or the planet gear. For this purpose a plurality of valves is provided each of which hydraulically operates an associated gear. For shifting from a then-present gear to a newly selected gear the valve associated with the then-present gear must be deactivated and that of the newly selected gear activated. If the transmission of torque is not to be interrupted, it is necessary that the valve associated with the newly selected gear be activated prior to the time that the valve associated with the then-present gear is deactivated. This process is known as a valve activation with positive overlap. Since the output torque of the engine is changed during a change in gear ratio it is necessary to provide means which prevent undesired torque peaks which might cause a jerk. It is also necessary to sense the speed at the input and the output of the gearing in order to terminate the shift process when the transmission input (engine output) and transmission output are operating in synchronism.
The operation of the gearing thus depends upon the system which controls its operation. This determines the wear and thus the life of the transmission as well as the smoothness of the transition, that is, for example, the comfort of the passengers of a vehicle.
Automatic electronically controlled transmission systems both for passenger vehicles and trucks are known. For example DT-OS No. 2,036,732 discloses a digital control system for automatic transmissions. This apparatus includes electronic sensors for sensing the engine speed and the output speed of the transmission. The sensed engine speed and transmission output signals are applied to Schmitt trigger circuits by means of which it can be determined whether the engine speed is lower than, equal to or higher than the transmission output speed. Depending on the so-determined operating condition, a brake may be applied to the transmission shaft or to the engine shaft or the supply of fuel to the engine may be regulated. It is the disadvantage of this system that cooperation between a purely mechanical synchronization member such as a brake with an electronically controlled fuel regulating system is difficult to achieve.
The suggestion has also been made that torque transmission during gear shift can be controlled by interrupting the fuel supply briefly during the shift operation. Specifically, this is to take place in response to the shift initiate signal during upshift and after a slight delay during downshifts. This method is relatively inaccurate and is suitable only for very simple drives.
Besides the above described systems which control the variation of torque by affecting the motor, other systems are known in which the torque transmission is affected by activation of the various couplings and brakes of the gearing itself. For this purpose the "jerk" or jolt of the vehicle is taken as reference and the couplings and brakes are so-controlled as to minimize such a jolt.
German DT-OS No. 2,124,024 discloses a system controlling automatic transmissions in which the couplings and brakes of the gearing are controlled so as to match a predetermined rate of change of motor speed with respect to time. This predetermined rate of speed is set in accordance with the motor speed, the motor torque, the direction of shift, the newly selected gear ratio, the temperature of the oil of the transmission and the load on the vehicle. These factors determine a reference value for the above mentioned jolt. The actual value of the jolt is taken to be the derivative with respect to time of engine speed. The actual and reference values are applied to a regulator which controls a pressure regulating member activating friction elements via an amplifier and an output control member. Peaks which might appear during the gear shift operation in either the torque or the oil pressure in the pressure regulating member are decreased by inserting delay elements, for example RC circuits in the electronic circuitry or oil atomizers in the hydraulic system. The disadvantage of these systems is that the actually present jolt is not sensed completely and that the jolt following the end of the shift process is not taken into consideration at all.
It has also been proposed that the actual jolt could be sensed, the jolt following the end of the shift process could be computed and the control of the couplings and brakes in the transmission could be carried out in such a manner that neither the actual jolt value during the shift operation nor the computed value at the end of the shift operation exceeds a predetermined reference value.
Finally it has been proposed to carry out the control of the couplings and brakes in the transmission in such a manner that the difference between the input and output speed of the gear is first reduced to a predetermined difference and that the speed thereafter be regulated in such a manner that a predetermined angle exists between the variation with respect to time of the transmission input and output speeds.
All the above described arrangements have the disadvantage that differences in the operating conditions between upshift and downshift and load and no load are not taken into consideration. Further, the torque transmission is changed by control of only one part of the drive system, that is either of the motor or, on the other hand, of the couplings and brakes associated with the transmission.